CATHODE RAY OSCILLOSCOPE (CRO)

INTRODUCTION:

A cathode ray oscilloscope, CRO or scope, is a versatile electronic instrument used in many fields of basic and applied research to measure time-dependent voltage signals. A CRO consists of a cathode ray tube, CRT (similar to a television picture tube), and associated circuits. Because an oscilloscope has very high resistance inputs (like a voltmeter), it draws very little current and thus usually does not disturb the circuit being studied. The oscilloscope is an essential part of several experiments in our introductory physics courses. Our scopes are not simplified for teaching but are versatile models suitable for use in research physics, engineering and medical laboratories.

WORKING : The cathode ray oscilloscope here contains three different parts. They are,

1. Electron gun2. Fluorescent screen3. Deflecting system

Here let us see the animation of the cathode ray oscilloscope which is used in the laboratory,

                          

Electron Gun: It is the one of the main parts in the animation of the cathode

ray oscilloscope. This consists oh header H, a cathode C, another electrode

called G and tow or three anodes A. G is an negative voltage with respect to C and controls the

number of electrons passing through its central hole from C to A is used to control the brightness.

The anode are at high positive voltage relative to C they accelerate the electrons along the highly evacuated tube and also focus into a narrow beam.

Fluorescent screen: A bright spot of the light is produced on the screen where the

beam hits it.

Deflecting system:Among the two pairs of defecting plates to which the potential differences can be applied.

The y-plates are horizontal but create a vertical electric field which deflects the beam vertically.The x-plates are presented in vertical direction and deflect the beam horizontally in horizontal direction.The p.d to create the electric field between the y-plates is applied to the y-input terminals in front of the CRO.The input is usually amplified by an amount that depends on the setting of the y-amp gain control before it is applied to the y-plates. The animation of the p.d applied to the y-plates with zero as the animation is given here,

                                    

CRO CONTROL BUTTONS:

The controls available on most oscilloscopes provide a wide range of operating conditions and thus make the instrument especially versatile. Since many of these controls are common to most oscilloscopes a brief description of them follows.

Power and Scale Illumination:  Turns instrument on and controls illumination of the graticule.

Focus:  Focus the spot or trace on the screen.

Intensity:  Regulates the brightness of the spot or trace.

VERTICAL AMPLIFIER SECTION:

Position:  Controls vertical positioning of oscilloscope display.

Sensitivity:  Selects the sensitivity of the vertical amplifier in calibrated steps.

Variable Sensitivity:  Provides a continuous range of sensitivities between the calibrated steps. Normally the sensitivity is calibrated only when the variable knob is in the fully clockwise position.

AC-DC-GND:  Selects desired coupling (ac or dc) for incoming signal applied to vertical amplifier, or grounds the amplifier input. Selecting dc couples the input directly to the amplifier; selecting ac send the signal through a capacitor before going to the amplifier thus blocking any constant component.

HORIZONTAL-SWEEP SECTION:

Sweep time/cm : Selects desired sweep rate from calibrated steps or admits external signal to horizontal amplifier.

Sweep time/cm Variable : Provides continuously variable sweep rates. Calibrated position is fully clockwise.

Position : Controls horizontal position of trace on screen.

Horizontal Variable : Controls the attenuation (reduction) of signal applied to horizontal amplifier through Ext. Horiz. connector.

TRIGGER

The trigger selects the timing of the beginning of the horizontal sweep.

Slope :   Selects whether triggering occurs on an increasing (+) or decreasing (-) portion of trigger signal.

Coupling :   Selects whether triggering occurs at a specific dc or ac level.

Source :   Selects the source of the triggering signal.

INT - (internal) - from signal on vertical amplifierEXT - (external) - from an external signal inserted at the EXT. TRIG. INPUT.LINE - 60 cycle trigger

Level :   Selects the voltage point on the triggering signal at which sweep is triggered. It also allows automatic (auto) triggering of allows sweep to run free (free run).

CIRCUIT DIAGRAM: